Pressure transducer utilizing non-lead containing frit

ABSTRACT

A piezoresistive sensor device and method for making the same are disclosed. The device comprises a silicon wafer having piezoresistive elements and contacts in electrical communication with the elements. The device further comprises a contact glass coupled to the silicon wafer and having apertures aligned with the contacts. The device also comprises a non-conductive frit for mounting the contact glass to a header glass, and a conductive non-lead glass frit disposed in the apertures and in electrical communication with the contacts. The method for making the device comprises bonding a contact glass to a silicon wafer such that apertures in the glass line up with contacts on the wafer, and filling the apertures with a non-lead glass frit such that the frit is in electrical communication with the contacts. The use of a lead free glass frit prevents catastrophic failure of the device in ultra high temperature applications.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation claiming priority under 35 U.S.C.§120 to U.S. patent application Ser. No. 12/686,990, filed Jan. 13,2010, which is continuation-in-part application claiming priority under35 U.S.C. §120 to U.S. patent application Ser. No. 12/455,922, entitled“METHOD AND APPARATUS FOR PREVENTING CATASTROPHIC CONTACT FAILURE INULTRA HIGH TEMPERATURE PIEZORESISTIVE SENSORS AND TRANSDUCERS”, filed 9Jun. 2009, which is a divisional application claiming priority under 35U.S.C. §121 to U.S. patent application Ser. No. 11/412,024, entitled“METHOD AND APPARATUS FOR PREVENTING CATASTROPHIC CONTACT FAILURE INULTRA HIGH TEMPERATURE PIEZORESISTIVE SENSORS AND TRANSDUCERS”, filed on26 Apr. 2006, all of which are hereby incorporated by reference in theirentirety as if fully set forth herein.

FIELD OF THE INVENTION

This invention relates to silicon on insulator leadless ultra hightemperature pressure transducers and more particularly to a method andapparatus for preventing catastrophic failure of contacts in suchtransducer.

BACKGROUND OF THE INVENTION

Some years ago, Kulite Semiconductor Products, Inc. (Kulite) hadreceived patents on the method of construction of high temperaturesilicon on oxide leadless pressure transducers. In our previous art, themethod for making the silicon-on-insulator sensor is described in U.S.Pat. No. 5,286,671 entitled “Fusion Bonding Technique for Use inFabricating Semiconductor Devices” issued on Feb. 15, 1994 to A. D.Kurtz et al. and assigned to Kulite the assignee herein, and the methodfor making the leadless high temperature transducer structure isdescribed in U.S. Pat. No. 5,955,771 entitled “Sensor for Use in HighVibrational Application and methods for Fabricating Same” issued on Sep.21, 1999 to A. D. Kurtz et al. and assigned to Kulite. See also U.S.Pat. No. 6,210,989 entitled “Ultra Thin Surface Mount Wafer SensorStructures and Methods for Fabricating the Same” issued on Apr. 3, 2001to A. D Kurtz et al. and assigned to the assignee herein. The devicesresulting from the methods described in the aforementioned patentspermitted the fabrication of structures which were suitable for use upto slightly over 600° C. However, it was found that at approximately620° C., or greater, there was a catastrophic failure in the electricalcontacts to the piezoresistive sensor network. Upon examination by theinventors herein, it was found that the use of the glass metal frit asso described in previous work, reacted with the metalized ohmic contactsand, in fact, dissolved them. In these devices the metalized contact wasformed by a layer of platinum silicide, titanium and platinum with theplatinum silicide being the layer immediately adjacent to the P+silicon. It was also found, however, that if a platinum wire wasdirectly bonded to the high temperature contact that no dissolution ofthe contact occurred when at temperatures as high as 700° C. Uponfurther observation, it was conjectured by the inventors that certain ofthe materials in the glass frit in and of themselves, were destroyingthe metal contact film layer and it was presumed that the presence oflead in the frit was the cause. In fact, the composition of the frit inthe aforementioned patents was typically about 60-80% lead, about 5-20%boron, about 5-20% silicon, with about 10-20% of either aluminum or zincadded. Originally, the reason for using a lead containing frit was tolower the melting point of the frit, thus enabling the use of a moresimple process to establish electrical continuity between the metalcontact layer and the pins on the header. However, it was discoveredthat at temperatures greater than 620° C. lead could interact withplatinum forming a liquidous, thereby dissolving the platinum anddestroying the contact. That meant that for high temperature operation,one would require a lead-free glass frit. Such glass frits arecommercially available from many sources and their compositions areapproximately 50% zinc, without any lead and with a mixture of boron andsilicon present. Other commercially available glass frits containstrontium instead of zinc. However, one reason such lead free glassfrits were deemed unsuitable for these operations was because theoriginal glass frit melting and softening points were considerablyhigher than the lead containing glass frits. When using such a lead-freefrit, the contact glass (as described in the aforementioned patents),namely borosilicate glass, would not withstand the new firingtemperatures required for the firing of the lead-free frits.Accordingly, the present invention resides in the recognition of theproblem and implementation of the solution to utilize lead-free glassfrits and glass to bond and otherwise utilize such lead-free glasses inthe formation of improved high temperature transducer devices.

SUMMARY OF THE INVENTION

Embodiments of the present invention are directed to a piezoresistivesensor device and a method for making a piezoresistive device. Thesensor device may comprise a silicon wafer having piezoresistiveelements and contacts in electrical communication with the elements. Thesensor device may further comprise a contact glass coupled to thesilicon wafer and having apertures aligned with the contacts. The sensordevice may also comprise a non-conductive frit for mounting the contactglass to a header glass, and a conductive non-lead glass frit disposedin the apertures and in electrical communication with the contacts. Themethod for making a piezoresistive sensor device, may comprise bonding acontact glass to a silicon wafer such that apertures in the glass lineup with contacts on the wafer, and filling the apertures with a non-leadglass frit such that the frit is in electrical communication with thecontacts. The use of a lead free glass frit prevents catastrophicfailure of the piezoresistive sensor and associated transducer in ultrahigh temperature applications.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a top plan view depicting a SOI leadless sensor according toan embodiment of the invention.

FIG. 2 depicts a schematic diagram showing an electrostatic bondingprocess according to an embodiment of the invention.

FIG. 3 depicts a perspective view of a SOI sensor according to anembodiment of the invention.

FIG. 4 depicts a sectional view of the sensor depicted in FIG. 3 withthe contact glass wafer electrostatically bonded to the silicon wafer.

FIG. 5 depicts a partial sectional view of a SOI leadless hightemperature sensor mounted on a header including header pins for use ina high temperature environment.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

As described herein, the use of lead-free glass frits in a hightemperature SOI leadless sensor gave rise to certain unanticipatedadvantages. Not only was it able to withstand much higher temperatures,but its expansion coefficient was much more closely matched to that ofsilicon (35 PPM/° C.) and the borosilicate glass versus (85 PPM/° C.)for the lead-bearing frit. In contrast, when the lead-bearing frit wasused to fill the holes in the contact glass, the difference in expansioncoefficients between the lead-bearing frit and the silicon borosilicatestructure gave rise to considerable elastic stress which degraded thedevice performance.

Furthermore, it was found that in order to use the high temperature, lowexpansion lead-free frit, a different contact glass was required capableof withstanding the higher melting point of the lead-free glass-metalfrit. It was discovered that glasses such as aluminum oxide-zincoxide-borosilicate glasses, not only had a higher melting point, butmatched the silicon expansion coefficients even better. Moreover, thisclass of glasses had a higher Young's modulus than the borosilicateglasses and, therefore, served to better isolate the silicon sensingelements from external thermal effects, leading to an enhanced device.Use of these various glass frits and contact glasses has enabled one tofabricate transducers which operate to temperatures well in excess of650° C. During and after exposure to these elevated temperatures thedevice continues to operate with excellent performance characteristics.Other glasses, such as alkaline-earth aluminosilicate glasses, canalternatively be used.

Bonding a flat surface of silicon to a flat surface of borosilicateglass is a relatively simple process and well known in the art (e.g.,using an electrostatic bond). However, to bond a layer of silicon to thealuminum oxide-zinc oxide-borosilicate, or alkaline-earthaluminosilicate, glass using the same technique, presented numerousproblems. These glasses have lower conductivity and fewer transportableions making the formation of an electrostatic bond more difficult.Furthermore, these glasses will only bond easily to an extremely smoothor ultra smooth surface. In the case when one desires to bond theseglasses to a P+ on top of silicon oxide region, there are furtherdifficulties. The P+ region as initially fabricated by conductivityselective etch, as in Kulite U.S. Pat. No. 5,286,671 entitled “FusionBonding Technique for Use in Fabricating Semiconductor Devices”, isrough in texture. Moreover, the areas of P+ used for contact regionswere rather large and because of the difference in expansion coefficientbetween the P+ silicon and the silicon dioxide to which it is affixed,they were frequently under stressed causing wrinkling or dimpling makingit almost impossible to seal those P+ regions to these glasses using anelectrostatic bond. Therefore, a different method of preparing the P+regions was necessary. Their extent was reduced and their surfaces weremade inherently smoother by continuing with the conductivity selectiveetch for a short time after the separation had occurred. This additionaltime in the conductivity selective etch tended to remove more of the P+silicon up to the most degenerative of the P++ layers, thus resulting ina smoother surface. These modifications in the procedures as well asoptimizing the geometry of the structures allowed for easier bonding ofthe Silicon wafer to the glass.

The sensor structure according to the embodiments of the presentinvention provides a more ideal mechanical configuration; being stiffer,and better thermally matched in terms of both filling glass-metal fritsand in terms of contact and header glasses used in the devicefabrication. This new mechanical structure results in more optimizedsensor performance characteristics across a wide temperature range ofoperation (cold to ultra hot). In fact, very accurate and very stablelow pressure devices, typically most affected by mechanical stresses,are now possible due to the present sensor construction.

Referring to FIG. 1, there is shown a top view of the surface geometryof an SOI leadless sensor employed in the present invention. It is notedthat the leadless sensor shown in FIG. 1 is the same sensor which isdescribed in U.S. Pat. No. 5,955,771 entitled “Sensors for Use in HighVibrational Applications and Methods for Fabricating Same” issued onSep. 21, 1999. In that patent FIG. 2 shows the top plan view of thesensor as depicted in FIG. 1 of the present invention. Certaindifferences will be explained. In any event, in order to understand thegeometry of FIG. 1, the following becomes pertinent. The pressure sensor(44) is approximately 100 mils by 100 mils or less and is fabricatedfrom two or more semiconductor wafers of silicon, or any other suitablesemiconductor wafer material. The transducer (44) is fabricated usingconventional wafer processing techniques which enable a number ofdielectrically isolated piezoresistive sensor elements such as (46),composed of highly doped P+ silicon to be formed on a semiconductormaterial using dielectric films of SiO₂. It is understood that a numberof such sensors can be made at the same time in a large substrate. Eachsensor element (46) is essentially a variable resistor comprising one offour legs of a Wheatstone bridge circuit with each of the respectiveresistances varying in proportion to an applied force or pressurethrough the transducer (44). The circuit nodes of the Wheatstone bridgeconsist of four oversized P+ diffuse silicon electrical contact areas orfingers (48). The fingers are mainly located in the non-activating areasof the transducer (44). The term “finger” is used to indicate that theareas (48) project from the sensor (46) to the metal contacts (50). Themetal contacts within the contact area are circular in shape and areeach approximately 10 mils in diameter. Each contact includes acentrally located area of high temperature platinum-titaniummetallization (50). In regard to the above noted patent FIG. 3 shows across-sectional view of the structure depicted in FIG. 1. As indicatedin the '771 patent, there is shown a bottom view of a cover which is tobe bonded to the transducer (44). The cover is fabricated from a glasssuch as pyrex. The cover to be electrostatically bonded without sealingglasses to the transducer (44). The apertures in the cover are filledwith a glass frit; typically the glass frit is made of Pyroceram a glassmaterial manufactured by Corning Glass Co. As indicated in the prior artdevices, the lead in this glass frit would react with the platinumcontacts, turning them into a liquid and thereby destroyingconductivity. This presented a significant problem. U.S. Pat. No.6,210,989 entitled “Ultra Thin Surface Mount Wafer Sensor Structures andMethod For Fabricating the Same” also shows transducer devices havingglass headers which include glass frits, applied in the apertures of theglass contact member. These structures also failed at temperatures above600° C.

Referring to FIG. 2, there is shown an electrostatic bonding processwhich molecularly joins the aluminosilicate glass to the smooth surface(11) of the SOI sensor wafer (10). The P+ region (11) as shown locatedon a layer of silicon dioxide. The process is performed on a hot plateat high temperatures. A metal plate (16) has a voltage applied by avoltage generator (15) which voltage is applied to the metal plate andalso to the silicon wafer (10). The pressure applied to the metal platewhich is positioned over the aluminosilicate glass wafer (12) enablesbonding of the glass wafer (12) to the surface of the P+ areas (11)associated with the sensor wafer (10). As seen the glass plate orcontact plate (12) has the contact aperture (18) located thereon. It isthese apertures (18) which eventually will be filled with a glass fritwhich does not contain lead. The metal plate (16) acts to spread theapplication or voltage across the entire contact glass wafer (12). Asindicated, the composition of the glass frit is utilized in theabove-noted patents which was the prior art contained between 60 to 80%lead, between 5-20% boron, and between 5-20% silicon, and with 10-20% ofeither aluminum or zinc which were added. These are the typicalstructures of the glass frit employed in the prior art. In any event, asindicated, for high temperature operation it has been discovered hereinthat a lead free glass frit is required. Such lead free glass frits arecommercially available and their compositions are approximately 50% zincand further containing a mixture of boron and silicon. Lead free glassfrits may also comprise strontium instead of zinc. These glasses werenever selected for uses in such devices because their melting points orsoftening points were considerably higher than the lead containing glassfrits. Thus, when using such a lead free frit the contact glass (12) asemployed in the devices described in the prior art patents, would notwithstand the increased firing temperatures required for the firing ofthe lead free frits. Contact glasses devoid of lead are available frommany manufacturers. These glasses typically contain silicon dioxide(SiO₂), aluminum oxide (Al₂O₃), boron oxide (B₂O₃), sodium oxide (Na₂O),magnesium oxide (MgO), arsenious oxide as well as zinc oxide and othercomponents. The lead free glasses differ according to the differentpercentages of such compositions. In any event, the major component ofsuch glasses is typically silicon dioxide with aluminum oxide also asubstantial component. The amount of silicon dioxide is generally in therange of about 25-70% of the composition with aluminum oxide being inthe range of about 15-30%. Boron oxide amounts are generally in therange of 0-10% with sodium oxide being in the range of about 0-5%. Theseglasses may also contain magnesium oxide. If magnesium oxide is present,it is normally in the range of 2-5%. These glasses may also containarsenious oxide, that if present, is the range of 10-20% accordingly.Arsenious oxide is being eliminated from alkaline earth glasses and oneuses CaO, BaO, lithium LiO2 or combinations thereof.

As indicated, to bond a piece of silicon as wafer (10) and the P+regions (11) to a glass contact wafer (12) which is totally devoid oflead using electrostatic bonding technique as depicted in FIG. 2presents considerable problems. The glass (12) has a lower conductivityand therefore has fewer transportable ions making the formation ofelectrostatic bond more difficult. Further, the glass (12) will onlybond to an ultra smooth surface. Further difficulties arise in the casewhere one desires to bond the glass wafer (12) to the P+ layer (11) ontop of the silicon oxide region. The P+ region (11) as initiallyfabricated by a conductivity selective edge is a rough surface thatbasically has a rough texture. Moreover the areas of P+ use or contactregions as seen in the prior patents are rather large and because of thedifference in expansion coefficient between the P+ silicon and thesilicon dioxide to which it is affixed, they were frequently understress and thereby caused wrinkling or dimpling. The wrinkling ordimpling made it almost impossible to seal those P+ regions to the glasswafer (12) using a conventional electrostatic or anoded bond. Therefore,a different method of preparing the P+ regions was necessary. Theirextent was reduced and the surfaces were made inherently smoother bycontinuing with the conductivity selective etching for a short timeafter separation had occurred. This additional time enabled theconductivity selective etch to remove more of the P+ silicon up to themost degenerate of the P+ layer, thus resulting in a smoother surface.By using these modifications and the procedures, it was possible to bondthe P+ region to the glass wafer. In addition, it was found that to usethe electrostatic bonding process with the glass wafer (12) that boththe temperature at which the bonding occurs as well as the voltageapplied had to be increased. In this way the glass wafer could beattached to the P+ regions of the semiconductor wafer (10). Thereafter,the use of the lead free glass frit to position the contacts in theapertures in the glass was possible resulting in an unanticipated andimproved structure.

With reference to FIG. 2, electrostatic bonding conditions using leadfree glasses are changed compared to the prior art electrostatic bondingtechniques. When using borosilicate glass the temperature of the bondingis at 400° C., and it takes about one hour to bond. The voltage used isabout 650 volts and the surface of the silicon can be semi-rough. Thisis according to prior art electrostatic bonding using the prior artglass. In accordance with an exemplary embodiment of the presentinvention, electrostatically bonding aluminum oxide-zincoxide-borosilicate glass or other lead free glass may be accomplished atlower temperatures, using lower voltage for a shorter period of time.For example, a lead free contact glass may be bonded by applying avoltage of approximately 300 volts or more at a temperature above 300°C. for 30 minutes or more. It is noted that the surface of the siliconhas to be smooth and of high quality and the geometry of the structuresmust be well controlled. When one uses an alkaline-earth aluminosilicateglass it is seen that the temperature is about 450° C., and that theirtime is about two hours. Furthermore, the voltage is at least 700 voltsand preferably about 900 volts. The surface of the silicon utilizingthat glass is of extremely high quality and ultra smooth. The preferredglass is the aluminum oxide-zinc oxide-borosilicate glass. The glass canbe utilized in conjunction with glass frits made from aluminumoxide-zinc oxide glass or the alkaline earth aluminosilicate glasses.Specifically, in alkaline earth glasses with no lead, sodium may also beeliminated. Alkaline earth metals such as CaO, BaO, and LiO₂ are used inthese glasses. The utilization of glass wafers and glass frits is wellknown as evidenced by the above-noted patents.

Referring to FIG. 3, there is shown a SOI leadless composite chip withan aluminosilicate contact glass wafer (36), which is to be attached tothe SOI leadless sensor by means of the electrostatic bond as depictedin FIG. 2. The ultra smooth surface quality of the P+ layer indicated byP+ platinum patterns (32) enables the electrostatic bonding process,which occurs between the aluminosilicate contact glass and the P+regions of the SOI sensor wafer. In FIG. 3 a silicon wafer (30) isdepicted. The silicon wafer has a layer of silicon dioxide (31) grown onthe surface. Deposited on the layer of silicon dioxide are P+ patternswhich include a peripheral rim (38) and the P+ contact patterns (32).The metallized contacts (33) are shown typically platinum or a platinumcompound. Also shown are the P type piezoresistors (34). As is known thealuminosilicate contact wafer (36) shown above has a cavity (35) toenable diaphragm deflection. The contact wafer (36) has contact throughholes (37). The contact through hole (37) aligns with each of themetallized contact areas and contact is made to the metalized areas bymeans of a lead free glass frit.

Referring to FIG. 4 there is shown the composite SOI leadless sensorchip with the contact holes filled with lead free glass metal frit (63).The presence of the aluminosilicate glass enables the ultra hightemperature filling process associated with a lead free glass metal fritfiring to take place. The glass metal frit may be the lead free glasscontaining gold or other high conductivity metal such as platinum. Asseen in FIG. 4, the silicon chip (62) is analogous to the chip (30) ofFIG. 3. The P type monocrystalline silicon piezoresistors (65) are shownand each of the resistors is directed to a metallized contact (64). Thestructure is deposited on a layer of silicon dioxide (63). Thealuminosilicate contact glass wafer (61) has the apertures whichcommunicate with the contact (64), the apertures being filled with alead free glass metal frit (60). The lead free glass metal frit andstructure of the sensor is depicted in FIG. 4.

Referring to FIG. 5 there is shown a high temperature leadless compositechip, as for example, the chip depicted in FIG. 4 mounted on a hightemperature header (73) using a non-conductive lead free glass frit. Asone can see the lead free glass frit, which is non-conductive (71)secures the sensor chip (70) to the header glass wafer (72). The headerglass wafer (72) may be a high temperature glass. In any event, as onecan see, the metal contact (76) on the sensor wafer is preserved duringthe high temperature mounting process and during any subsequent deviceoperation. This is due to the removal of lead from the contactinterface. The aluminosilicate contact glass makes possible the hightemperature mounting process. As seen in FIG. 5, the apertures which arefilled with the lead free glass metal frit are now directed to headerpins (74). There are normally four header pins associated with aWheatstone bridge which as one can ascertain a Wheatstone bridge hasfour active contacts. An applied pressure (77) is applied to the sensorin the active area causing the piezoresistors to respond producing avoltage proportional to the applied pressure. Thus, as seen, there isshown a high temperature sensor transducer which provides anunanticipated, unexpected result in using lead free glasses and leadfree frits to form high temperatures sensors and transducers. It will beapparent to those skilled in the art that modifications and variationsmay be made in the apparatus and process of the present inventionwithout departing from the spirit or scope of the claims. It is intendedthat the present invention cover the modification and variations of thisinvention provided they come within the scope of the amended claims andtheir equivalents.

1. A leadless sensor mounting method, comprising: bonding anon-conductive contact cover to a sensor device, such that aperturesdefined within the contact cover align with metalized electricalcontacts on the sensor device; disposing a conductive, non-leadcontaining frit within the apertures to facilitate electricalcommunication with the metalized electrical contacts on the sensordevice; and mounting the contact cover onto a header assembly utilizinga non-conductive, non-lead containing frit as an adhesive, wherein theapertures of the contact cover align and electrically communicate withheader pins on the header assembly.
 2. The method of claim 1, furthercomprising simultaneously curing the conductive, non-lead containingfrit and the non-conductive, non-lead containing frit.
 3. The method ofclaim 1, wherein bonding comprises electrostatically bonding the sensordevice to the contact cover at a temperature above approximately 300° C.4. The method of claim 1, further comprising depositing an oxide layeron the sensor device prior to bonding.
 5. The method of claim 1, whereinthe sensor device is “upside-down” bonded to the contact cover.
 6. Themethod of claim 1, wherein the contact cover is electrostatically bondedto the sensor device.
 7. The method of claim 1, wherein the sensordevice is a piezoresistive sensor.
 8. The method of claim 1, wherein theconductive, non-lead containing frit is a glass-metal frit.
 9. Themethod of claim 1, wherein the non-conductive, non-lead containing fritis a glass frit.
 10. The method of claim 1, wherein the contact cover isa glass contact cover.
 11. A leadless sensor device, comprising: asensor device, comprising at least one electrical contact; anon-conductive contact cover bonded to the sensor device, wherein thecontact cover defines apertures aligned with each of the electricalcontacts on the sensor device, and further wherein a conductive,non-lead containing frit is disposed within each of the apertures; and aheader assembly bonded to the contact cover, wherein a non-conductive,non-lead containing frit is disposed therebetween.
 12. The leadlesssensor device of claim 11, wherein the sensor device is “upside-down”bonded to the contact cover.
 13. The leadless sensor device of claim 11,wherein the sensor device is a piezoresistive sensor.
 14. The leadlesssensor device of claim 11, wherein the conductive, non-lead containingfrit is a glass-metal frit.
 15. The leadless sensor device of claim 11,wherein the non-conductive, non-lead containing frit is a glass frit.16. The leadless sensor device of claim 11, wherein the electricalcontacts comprise platinum metallization.
 17. The leadless sensor deviceof claim 11, wherein the sensor device is fabricated from two or moresemiconductor wafers of silicon.
 18. The leadless sensor device of claim11, wherein the contact cover is a glass contact cover.